If you're going to grad school in physics or astronomy, you are going to have to take the physics GRE. It's hard, and to do well you will have to study. The review books are universally considered to be terrible, so you are better off with studying the practice tests in depth and using study guides from the internet. The links below should help you out.

ETS official site

ETS GRE site

Practice tests

The newer tests are generally considered to be easier (linked to Stanford SPS).
GRE Physics Test: 1986
GRE Physics Test: 1992
GRE Physics Test: 1996
GRE Physics Test: 2001

Independent sites

GREphysics.net: solutions to all problems from the above 4 tests (possibly the most useful site on the web)
physicsGRE.com: physics forum

University GRE sites

Review sheets and tips.
Stanford SPS
Oregon State Physics Department
Harvard SPS

Review sheets from SPS member Isaac Storch:
pdf format
tex source file
If you have review sheets and would like to share them - or if you edit/ improve upon Isaac's - please email sps(at)physics.ucsb.edu and we'll post them for others to use.